There are two types of corn-to-ethanol processing, wet milling and dry milling. The central difference between the two processes is how they initially treat the grain. In wet milling, the corn grain is steeped in water, and then separated for processing in the first step. Dry milling, which is more common, requires a different process.
The corn dry-milling process utilized in the production of ethanol is well known. Post fermentation and distillation, the spent grains are further processed prior to being sold as distiller's grains products. Ethanol plants typically treat whole stillage from the beer distillation column via centrifugation to produce wet cake and thin stillage then further treat the thin stillage stream by subjecting it to multiple effect evaporation to increase the solids and recover the distillate for return use in the process. As solids increase the thin stillage is typically referred to as syrup. The syrup is typically combined with wet cake or distiller's dry grains and sold as animal feed. These processes are well known in the industry and are generally employed in plant design in the industry.
In an effort to take advantage of co-product streams, many plants have added oil removal processes in which the thin stillage (sometimes referred to as syrup) is subjected to processes, such as centrifugation or extraction, to remove corn oil from the thin stillage. For example, the application of centrifuges to the separation of corn oil from thin stillage is in wide use in the fuel ethanol industry. While the theoretical yield of oil per bushel of processed corn is about 2 pounds per bushel, many of the commercial installations fall far short of this. Thus there remains a need to improve the process to maximize the oil yield of the process.
Recently there has been effort directed at increasing the value model of the corn-to-ethanol production process by extracting the oil from the thin stillage byproduct. U.S. Pat. No. 7,601,858 describes a mechanical method of separating the oil from concentrated thin stillage, referred to as the “syrup”, using a disk stack centrifuge. U.S. Pat. Appl. No. 2008/0176298 teaches the use of an alkyl acetate solvent for extraction of corn oil in an ethanol production process.
Although the prior art references are effective, there is still opportunity to improve the oil extraction process to further maximize the commercial value of the process. Of particular interest are technologies which do not require the adoption of capital expenditures to implement a new mechanical solution and/or significant process change such as the use of an extraction solvent that requires recycling.